Tube straightening method



Nov. 15, 1966 H, CRAWFORD 3,285,047

TUBE STRAIGHTENING METHOD Original Filed March 27, 1961 United States Patent O 3,285,047 TUBE STRAIGHTENING METHOD Joseph Howard Crawford, Winnebago, 11]., asszgnor to General Electric Company, a corporation of New York Continuation of application Ser. No. 98,542, Mar. 27, 1961. This application Dec. 18, 1964, Ser. No. 421,748 1 Claim. (Cl. 72-79) This invention relates to a tube straightening method and, more particularly in one form, to a method for making very straight thin walled coated tubular cathodes. This is a continuation application of application Serial Number 98,542, filed March 27, 1961, now abandoned, and assigned to the assignee of the present invention.

The development of new, strong, difiicult-to-work metal alloys suitable for high temperature use has introduced problems relating to the working, machining, cutting, drilling and grinding of such alloys. Electrochemical and electrodischarge type methods and apparatus have been developed to facilitate the shaping of articles made of such alloys. There are a large number of publications which describe method and apparatus suitable for electrolytic type material removal.

Referring particularly to the production of long, narrow, straight passages produced by electrochemical material removal, one of the major factors in successful operation is the degree of straightness of the cathode used. Particularly useful in producing such passages are long, straight, tubular coated electrodes. A typical passage of this type is 0.050 in diameter and 8" long with a deviation of no more than 0.005" of true location.

Prior to this invention it had been proposed to produce very straight tubing, particularly of the thin Wall variety, by heating the tube while it was being stretched. For example, this was accomplished by weighting one end of a tube suspended through a furnace. However, this method was too difi'icult and time consuming to operate on a production basis. Furthermore, the temperature to which the tubing must be raised in order to soften such tubing can cause serious difliculties, costly to overcome. For example, it can deteriorate the dielectric type coatings required on electrodes useful for electrochemical material removal; it can cause heat damage to many metallic tube materials; and it can cause oxidation of the surface of an uncoated tube requiring costly and diflicult scale removal processing.

Although mechanical methods, such as described in US. Patents 1,326,981 and 2,938,563, result in spiral working as does the method of this invention, such prior methods and apparatus cause tube surface damage. For example, damage is caused through indentation or scoring in the surface of a bare tub or in a dielectric coating. Such imperfections cause premature breakdown of a ielectric coating during operation or become early fatigue points or stress risers in thin wall tubing. Furthermore, working of the tubing in local areas according to prior methods and apparatus can cause collapse of very thin wall tubing.

Therefore, it is a principal object of this invention to provide a method for straightening thin wall tubing without damaging its outer surface.

Another object is to provide a method for straightening coated thin wall tubing without damage to the coating.

Still another object is to provide a method for making straight coated tubular cathodes of improved life.

These and other objects and advantages will become more apparent from the following detailed description and the drawing in which:

FIG. 1 is an isometric, partially diagrammatic view of a preferred form of the apparatus useful in the practice of this invention;

ice

FIG. 2 is a similar view of another embodiment of such apparatus, and

FIG. 3 is a fragmentary sectional view of another form of the apparatus.

The present invention provides a method for straightening metal tubing which comprises passing the tubing axially through a bending member having smooth continuous walls. At the same time, the tubing is bent to provide a bending stress in the tubing to at least exceed the elastic limit of the tubing and relative radial rotation between the tubing and the bending member.

It was unexpectedly discovered that by providing relative axial movement of metal tubing along a particularly curved path within a smooth, continuous tubular passage while the tubing and tubular passage were rotating with respect to one another around substantially the same axis, an unusually straight tube of undamaged surface is obtained. In this way, generally uniform and continuous working was achieved over the entire area of the tubing being straightened. Furthermore, because the tubular passage had a smooth, continuous internal surface or wall, no damage occurred to the tubing surface adjacent the inner surface of the tubular passage. This was true of coated as well as uncoated tubing. As will be shown later in detail, such tubular passage can consist of a guide tube, a guide tube within a housing or a housing in which a guiding tubular passage has been produced.

Referring now to FIG. 1, a preferred form of the tube straightening apparatus used in this invention is shown generally at 10. The apparatus includes a tubular guide member 12 having a smooth, continuous inner surface. Tubular guide member 12 is carried by a housing 14 which can be a single member or can be made in sections such as halves 14a and 14b for convenience of access and manufacture. This tubular guide member 12 can be carried, such as by clamping, welding, brazing, pressing, etc. into an appropriate holder or indentation, by housing 14.

Continuous tubular guide member 12 has two end portions 120, preferably straight, in substantial alignment with each other along line 16, and an intermediate portion 12b deflected from line 16 along a smoothly curved path for a distance B and at a deflection A. The distances A and B are functions of the material, wall thickness and diameter of the tubing as well as the relative axial and rotational motion between the tubing and the tubular guide member. Distances A and B and such relative motion are selected to provide, at a minimum, suflicient stress in the tubing to at least exceed the elastic limit of the material of the tubing. The term elastic limit is defined as the maximum stress that a material will withstand without permanent deformation.

During operation, tubing 11 to be straightened passes within tubular guide member 12 as shown by arrow 15, in FIGS. 1, '2 and 3, through an appropriate motion producing means attached to either or both tubular guide member and the tubing. For example, one end of the tubing can be secured to a movable piston while the tubular guide member is held stationary. In another form, the tubular guide member 12 can be moved along tubing 11; or both the tubular guide member and the tubing 11 can be moved axially in relation to one another.

Concurrent with the axial motion occurring between tubing 11 and tubular guide member 12 is a relative rotating motion between tubing 11 and the tubular guide member 12 such as shown at 17 in FIGS. 1, 2 and 3. Such relative motion can be achieved by rotating the tubular guide member 12 or by rotating the tubing being straightened or both. Rotation can be accomplished, for example, by attaching a rotating means such as individual motors, gears and pulley systems to either or both of the components being rotated. In another example, by securing one end of the tubing 11 to a rotating portion of a lathe and securing housing 12 to a traversing lathe member, both the rotating and axial motions described above can be accomplished with accurate control of both motions.

In other forms, the tubular guide member 12 can be suspended, as in FIG. 2 between support means 18 which can be bearings if the tubular guide member is rotated or clamps if it is stationary.

As shown in FIG. 3, a tubular guide member in the form of a housing including a tubular passage can be produced from a substantially solid housing block such as 14. In the arrangement of FIG. 3, a tubular alignment member 13 can be attached'to housing block 14 to direct the tubing to be straightened in and out of the tubular guide member passage.

In order to avoid drawing or extruding the tube during straightening, the tubular guide member 12 must have an inside diameter larger than the outside diameter of the tubing, or tubing plus coating if coated tubing is being straightened. Although the inside diameter of the tubular guide member in relation to the outside diameter of the tubing being straightened is not a specifically critical limitation the tolerance should not be great enough to cause severe misalignment and local working. For example, a maximum diameter difference of about 0.015" is preferred for tubing of about 0.040" outside diameter.

By passing tubing 11 and tubular guide member 12 axially with respect to each other while producing relative rotating motion between the two, unusually straight tubing of unmarred surface can be obtained. These results are brought about by the smooth, continuous inner surface of tubular guide member 12 which works uni formly the entire tubing wall along portion B of the tubular guide member. The smooth, continuous inner surface of the guide member 12 does not include edges, protuberances and the like which can produce imperfections such as spiral score markings in the surface of the tubing or its coating. Such imperfections can cause premature failure of thin wall tubing or premature breakdown of a dielectric coating such as that required on cathode tubes used in high current density operating electrochemical material removal processes.

In one form of the method of the present invention, the apparatus of FIG. 1 was used to straighten 14" long titanium tubing coated with a tetrafluo rethylene material.

The outside diameter of the coated tube was 0.038" and dimension B was 1.850. During operation of the method, the tubing was held stationary and the tubular guide member was rotated at a speed of about 300 r.p.m. as the tubing was passed through the tubular guide member at a rate of about 0.003" per revolution. The relative speed of rotation of the members is not important. However, the feed rate of tubing per revolution must be adjusted for the tubing material and the thickness of coating in relation to dimensions A and B. In this example, for a coating thickness of about 0.00 "-0.00 on titanium tubing of 0.006 Wall thickness, it is preferred to use a tubing feed rate of about 0.003" per revolution. By the apparatus used in the practice of this invention, -95% of the tubing straightened had a deviation from true straightness of less than 0.001" in the 14" length.

The present invention is particularly useful in straightening tubing of about 0.1" or less outside diameter without the use of heat or stretching forces. Coated tubes or cathodes for electrochemical material removal processes can be straightened as a final operation in their manufacture without damaging the coating surface. Furthermore, with many materials, the cold working performed on the tubing material improves its physical properties.

Although this invention has been described in connection with particular examples, these are meant to be exemplary of rather than limitations on the invention. It will be understood by those skilled in the art the variations and modifications of which this invention is capable.

What is claimed is:

A method for straightening coated metal tubing of about 0.1" or less outside diameter, comprising the steps of:

passing the tubing at a rate of about 0.003" per minute axially through a bending member having smooth, continuous walls; and, concurrently, bending the tubing during passage through the bending member to provide a bending stress in the tubing to slightly exceed the elastic limit of the tubing; while,

applying relative rotation between the tubing and the bending member.

References Cited by the Examiner UNITED STATES PATENTS 149,666 4/ l 874 Mallett 72-79 1,229,981 6/ 1917 Lehmann 7279 2,369,329 2/ 1945 Wennerberg 72-79 CHARLES W. LANHAM, Primary Examiner. R. D. GREFE, Assistant Examiner. 

